Silicon quantum dot coated microspheres for microfluidic refractive index sensing

The electromagnetic resonances of optical microspheres—the so-called whispering gallery modes (WGMs)—can be used for refractometric sensing of surrounding fluids. Microspheres are attractive because they offer high sensitivity and can be utilized with fluorescent dyes or quantum dots. One issue with microspheres, however, is that they are difficult to integrate into microfluidic systems. Here, we develop a microfluidic structure that permits sensing applications using a single microsphere in a capillary. To achieve this, a microsphere formed on the end of a tapered fiber was first coated with fluorescent silicon quantum dots (QDs). The sphere was then inserted into a microcapillary and the fluorescence WGMs were monitored as different fluids were pumped through the channel. The sensitivity and detection limits for this sphere-in-a-capillary device were measured for several different QD film thicknesses and for two different microsphere sizes. Because of the relatively high-visibility mode structure, the sensitivity and detection limit can be defined by Fourier analysis of the free spectral range and WGM spectral shifts.